Packet Tracer is just a simulation, so No, it is not possible to actually change the IOS. The simulation does allow you to use TFTP to copy a different version to the router, and use the "boot system" command to set the new version to load on the next reboot, but once you reload the router, it ignores the command.

Correction: You can upgrade the IOS on the 2600XM router, and this does "stick" after reboot, as this is the way you get IPv6 functionality to work on the 2600XM routers. Howerver, the upgrade only add some commands - not the full feature set of the upgraded IOS.